The present invention relates to a composite catalyst suitable for use in solid polymer electrolyte type fuel cells and to processes for producing the catalyst.
A solid polymer electrolyte type fuel cell (PEFC) is an apparatus which employs a solid ion-exchange membrane as an electrolyte and in which a fuel (e.g., hydrogen gas) and an oxidizing agent (e.g., oxygen gas) are supplied respectively to the anode and the cathode to electrochemically react these feed materials on the catalyst surfaces, thereby obtaining an electric power.
For example, in the case of using hydrogen gas and oxygen gas as a fuel and an oxidizing agent, respectively, the electrochemical reactions occurring at the electrodes are as follows.
Anode: H2xe2x86x922H++2exe2x88x92
Cathode: 1/202+2H++2exe2x88x92xe2x86x92H2O
Overall reaction: H2+1/202xe2x86x92H2O
As the formulae given above show, the reactions at the anode and cathode necessitate the feed of oxygen and hydrogen gases and the transfer of protons (H+) and electrons (exe2x88x92). Namely, all these reactions proceed only at sites where the feed and transfer are satisfied simultaneously.
An electrode for fuel cells is diagrammatically shown in FIG. 25. This electrode has a catalyst layer 261 and a gas diffusion layer 263. The catalyst layer 261 is constituted, for example, of a mixture of carbon particles supporting catalyst metal 265 and a solid polymer electrolyte 271. The particles 265 and the electrolyte 271 are three-dimensionally distributed so that the layer has pores 267 in inner parts thereof, i.e., is porous. The gas diffusion layer 263 is constituted of a porous electro-conductive material 269, which comprises, e.g., a porous carbon paper. This electrode is bonded to a cation-exchange membrane 275 to thereby fabricate a fuel cell. This gas diffusion layer 263 provides not only to secure passageways for transferring the oxygen gas and hydrogen gas fed externally as reactants to a surface of the catalyst layer 261 but also to provides passageways for discharging the water yielded in the catalyst layer of the cathode from a surface of the catalyst layer 261 to the outside of the cell. On the other hand, in the catalyst layer 261, the carbon supporting catalyst metal 265 forms electro-conductive channels and the solid polymer electrolyte 271 forms proton-conductive channels. The pores 267 function not only as feed channels through which the oxygen or hydrogen gas transferred to a surface of the catalyst layer 261 is supplied to inner parts of the catalyst layer but also as gas diffusion channels for discharging the water yielded in inner parts of the catalyst layer (cathode) to a surface of its layer. These three kinds of channels are three-dimensionally distributed in the catalyst layer 261 to form innumerable sites where gas transfer can occur simultaneously with the transfer of protons (H+) and electrons (exe2x88x92). Thus, sites for the electrode reactions are provided.
Incidentally, the solid polymer electrolyte 271 comprising a cation-exchange resin used as a proton conductor shows satisfactory proton conductivity only when it is in a hydrous state. Consequently, for preventing the solid polymer electrolyte 271 from drying, a technique is being used in which the gases to be supplied to the anode and cathode are humidified before being supplied. However, this technique has aroused a problem that when the solid polymer electrolyte type fuel cell is operated at a high current density, water floods on the surface of the catalyst layer 261 and in the pores 267 to inhibit gas diffusion, resulting in a considerably reduced output. Especially, this problem is tend to be occurred because the reaction yields water in it.
A technique generally employed for avoiding the water flooding caused by water generation and gas humidification is to impart water repellency to an electrode by incorporating polytetrafluoroethylene (PTFE) particles 273, which exhibits excellent hydrophobic property, together with catalyst particles 265 in the formation of a catalyst layer or by applying PTFE particles 273 to the surface of an porous electro-conductive material 269. However, in order to prevent water flooding enough in the electrode during high-current-density operation, it is necessary to incorporate PTFE particles 273 in an even larger amount to thereby enhance water repellency. Although highly water-repellent, the PTFE particles 273 do not have gas-diffusing properties, not to mention electron conductivity and proton conductivity. Because of this, mixing of large amount of the PTFE particles block electron-conductive channels, proton-conductive channels, and gas diffusion channels, arousing a problem that the output of the fuel cell is reduced rather than increased. In addition, the pores 267 formed among the catalyst particles 265 are partly clogged by the cation-exchange resin and, as a result, the gas diffusion channels are partly blocked to prevent a reactant gas from being supplied to the whole catalyst layer 261 including minute regions thereof. Namely, there has been a problem that the degree of catalyst utilization is low and the fuel cell has a high concentration overvoltage and hence a low cell voltage.
A first object of the invention is to provide a composite catalyst capable of high electron conductivity, proton conductivity, and gas-diffusing properties while preventing water flooding. A second object of the invention is-to provide gas-diffusing properties while improving proton conductivity by adhering a cation-exchange resin to the surface of a catalyst to thereby improve the degree of catalyst utilization.
The invention provides a composite catalyst characterized by comprising catalyst particles and, adherent to the surface thereof, a porous or net-form cation-exchange resin and/or a porous or net-form hydrophobic polymer.
The adhesion of a cation-exchange resin to the surface of catalyst particles improves the proton conductivity of the catalyst surface. Since the porous or net-form cation-exchange resin is adherent to the surface of the catalyst particles so as to leave the catalyst surface partly exposed, reactant gases can reach the surface of the catalyst particles through the pores or net openings of the resin, whereby the degree of catalyst utilization can be improved. The adhesion of a hydrophobic polymer to the surface of catalyst particles is effective in preventing water flooding in an electrode. Furthermore, since the resin or polymer is adherent to the surface of the catalyst Particles so as to leave the catalyst surface partly exposed, electron conductivity, proton conductivity, and gas-diffusing properties can be secured.
In the case of the composite catalyst having both a porous or net-form cation-exchange resin and a porous or net-form hydrophobic polymer which are adherent to the surface of the catalyst particles, it is a matter of course that the effects brought about when the resin and polymer are used alone can be obtained simultaneously. Furthermore, in the invention, the catalyst particles preferably have a porous or net-form cation-exchange resin adherent to the surface thereof and further have a hydrophobic polymer adherent thereto in the pores or net openings of the cation-exchange resin so as to leave the surface of the catalyst particles partly exposed. Consequently, this hydrophobic polymer is in close contact with the cation-exchange resin and the catalyst particles. This promotes to form site where transfer of protons (H+) and transfer of electrons (exe2x88x92) can occur simultaneously, whereby the output of a fuel cell can be further improved.
The composite catalyst of the invention can be produced by a process which comprises the steps of: adhering a solution (a) prepared by dissolving a cation-exchange resin and/or a hydrophobic polymer in a solvent to the surface of catalyst particles; and subsequently undergoing phase separation resulting in removing the solvent from the solution (a). It is preferable that the step of phase separation is occurred by extracting the solvent from solution (a) with solution (b) which is insoluble for the resin or polymer in the solution (a) and is compatible with solvent.
In this process, the solution (a) prepared by dissolving a cation-exchange resin and/or a hydrophobic polymer in a solvent first adheres to the surface of catalyst particles. Thereafter, these catalyst particles are brought into contact with a solution (b) which is insoluble for the resin or polymer and is compatible with the solvent. As a result, the solvent is immediately replace by solution (b) Namely, the resin or polymer molecules coagulate resulting in formation a porous or net-form cation-exchange resin and/or hydrophobic polymer on the surface of catalyst particles.
The present invention relates to:
(1) A composite catalyst which comprises a catalyst particle and at least one member member selected from the group consisting of a porous or net-form cation-exchange resin and a porous or net-form hydrophobic polymer, wherein the resin and polymer exist on the surface of the catalyst particle.
(2) The composite catalyst of (1), wherein the catalyst particle preferably comprises a carbon supporting catalyst metal (i.e., the catalyst particle comprises a carbon particle and a catalyst metal provided thereon).
(3) The composite catalyst of (1), wherein the hydrophobic polymer is a fluoropolymer.
(4) The composite catalyst of (1), wherein the amount of the adherent hydrophobic polymer is from 0.01 to 30 wt % based on the catalyst particle.
(5) The composite catalyst of (1), wherein the amount of the adherent cation-exchange resin is from 1.0 to 100 wt % based on the catalyst particle. 100 wt % based on the catalyst particle.
(6) A process for producing a composite catalyst which comprises the steps of:
adhering a solution (a) prepared by dissolving at least one member selected from the group consisting of a cation-exchange resin and a hydrophobic polymer in a solvent to the surface of a catalyst particle; and
subsequently undergoing phase separation resulting in removing the solvent from the solution (a).
(7) The process of (6), wherein the step of phase separation is occurred by extracting the solvent from the solution (a) with an solution (b) which is insoluble for the resin or polymer in the solution (a) and is compatible with the solvent.
(8) A process for producing an electrode for fuel cells which comprises:
adhering a solution (a) prepared by dissolving at least one member selected from the group consisting of a cation-exchange resin and a hydrophobic polymer in a solvent to the surface of a catalyst particle;
subsequently undergoing a phase separation of solution (a) so as to remove the solvent to produce a composite catalyst; and
pressing a mixture comprising the composite catalyst to form a catalyst layer.